Method and Apparatus for an Adaptive Ladar Receiver

ABSTRACT

Disclosed herein are various embodiments of an adaptive ladar receiver and associated method whereby the active pixels in a photodetector array used for reception of ladar pulse returns can be adaptively controlled based at least in part on where the ladar pulses were targeted. Additional embodiments disclose improved imaging optics for use by the receiver and further adaptive control techniques for selecting which pixels of the photodetector array are used for sensing incident light.

CROSS-REFERENCE AND PRIORITY CLAIM TO RELATED PATENT APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 15/590,491, filed May 9, 2017, and entitled “Method andApparatus for an Adaptive Ladar Receiver”, which is acontinuation-in-part of U.S. patent application Ser. No. 15/430,179,filed Feb. 10, 2017, and entitled “Adaptive Ladar Receiving Method”,which claims priority to U.S. provisional patent application 62/297,112,filed Feb. 18, 2016, and entitled “Ladar Receiver”, the entiredisclosures of each of which are incorporated herein by reference.

This patent application is also a continuation-in-part of U.S. patentapplication Ser. No. 15/430,192, filed Feb. 10, 2017, and entitled“Adaptive Ladar Receiver”, which claims priority to U.S. provisionalpatent application 62/297,112, filed Feb. 18, 2016, and entitled “LadarReceiver”, the entire disclosures of each of which are incorporatedherein by reference.

This patent application is also a continuation-in-part of U.S. patentapplication Ser. No. 15/430,200, filed Feb. 10, 2017, and entitled“Ladar Receiver with Advanced Optics”, which claims priority to U.S.provisional patent application 62/297,112, filed Feb. 18, 2016, andentitled “Ladar Receiver”, the entire disclosures of each of which areincorporated herein by reference.

This patent application is also a continuation-in-part of U.S. patentapplication Ser. No. 15/430,221, filed Feb. 10, 2017, and entitled“Ladar System with Dichroic Photodetector for Tracking the Targeting ofa Scanning Ladar Transmitter”, which claims priority to U.S. provisionalpatent application 62/297,112, filed Feb. 18, 2016, and entitled “LadarReceiver”, the entire disclosures of each of which are incorporatedherein by reference.

This patent application is also a continuation-in-part of U.S. patentapplication Ser. No. 15/430,235, filed Feb. 10, 2017, and entitled“Ladar Receiver Range Measurement Using Distinct Optical Path forReference Light”, which claims priority to U.S. provisional patentapplication 62/297,112, filed Feb. 18, 2016, and entitled “LadarReceiver”, the entire disclosures of each of which are incorporatedherein by reference.

INTRODUCTION

It is believed that there are great needs in the art for improvedcomputer vision technology, particularly in an area such as automobilecomputer vision. However, these needs are not limited to the automobilecomputer vision market as the desire for improved computer visiontechnology is ubiquitous across a wide variety of fields, including butnot limited to autonomous platform vision (e.g., autonomous vehicles forair, land (including underground), water (including underwater), andspace, such as autonomous land-based vehicles, autonomous aerialvehicles, etc.), surveillance (e.g., border security, aerial dronemonitoring, etc.), mapping (e.g., mapping of sub-surface tunnels,mapping via aerial drones, etc.), target recognition applications,remote sensing, safety alerting (e.g., for drivers), and the like).

As used herein, the term “ladar” refers to and encompasses any of laserradar, laser detection and ranging, and light detection and ranging(“lidar”). Ladar is a technology widely used in connection with computervision. In an exemplary ladar system, a transmitter that includes alaser source transmits a laser output such as a ladar pulse into anearby environment. Then, a ladar receiver will receive a reflection ofthis laser output from an object in the nearby environment, and theladar receiver will process the received reflection to determine adistance to such an object (range information). Based on this rangeinformation, a clearer understanding of the environment's geometry canbe obtained by a host processor wishing to compute things such as pathplanning in obstacle avoidance scenarios, way point determination, etc.However, conventional ladar solutions for computer vision problemssuffer from high cost, large size, large weight, and large powerrequirements as well as large data bandwidth use. The best example ofthis being vehicle autonomy. These complicating factors have largelylimited their effective use to costly applications that require onlyshort ranges of vision, narrow fields-of-view and/or slow revisit rates.

In an effort to solve these problems, disclosed herein are a number ofembodiments for an improved ladar receiver and/or improved ladartransmitter/receiver system. For example, the inventors disclose anumber of embodiments for an adaptive ladar receiver and associatedmethod where subsets of pixels in an addressable photodetector array arecontrollably selected based on the locations of range points targeted byladar pulses. Further still, the inventors disclose example embodimentswhere such adaptive control of the photodetector array is augmented toreduce noise (including ladar interference), optimize dynamic range, andmitigate scattering effects, among other features. The inventors showhow the receiver can be augmented with various optics in combinationwith a photodetector array. Through these disclosures, improvements inrange precision can be achieved, including expected millimeter scaleaccuracy for some embodiments. These and other example embodiments areexplained in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example embodiment of a ladartransmitter/receiver system.

FIG. 1B illustrates another example embodiment of a ladartransmitter/receiver system where the ladar transmitter employs scanningmirrors and range point down selection to support pre-scan compression.

FIG. 2 illustrates an example block diagram for an example embodiment ofa ladar receiver.

FIG. 3A illustrates an example embodiment of detection optics for aladar receiver, where the imaging detection optics employ a non-imaginglight collector.

FIG. 3B illustrates another example embodiment of detection optics for aladar receiver, where the afocal detection optics employ a non-imaginglight collector.

FIG. 4 illustrates an example embodiment of imaging detection optics fora ladar receiver, where the imaging detection optics employ an imaginglight collector.

FIG. 5A illustrates an example embodiment of a direct-to-detectorembodiment for an imaging ladar receiver.

FIG. 5B illustrates another example embodiment of a direct-to-detectorembodiment for a non imaging ladar receiver.

FIG. 6A illustrates an example embodiment for readout circuitry within aladar receiver that employs a multiplexer for selecting which sensorswithin a detector array are passed to a signal processing circuit.

FIG. 6B illustrates an example embodiment of a ladar receiving methodwhich can be used in connection with the example embodiment of FIG. 6A.

FIG. 7A depicts an example embodiment for a signal processing circuitwith respect to the readout circuitry of FIG. 6A.

FIG. 7B depicts another example embodiment for a signal processingcircuit with respect to the readout circuitry of FIG. 6A.

FIG. 8 depicts an example embodiment of a control circuit for generatingthe multiplexer control signal.

FIG. 9 depicts an example embodiment of a ladar transmitter incombination with a dichroic photodetector.

FIG. 10A depicts an example embodiment where the ladar receiver employscorrelation as a match filter to estimate a delay between pulsetransmission and pulse detection.

FIG. 10B depicts a performance model for the example embodiment of FIG.10A.

FIGS. 11A-11E depict example embodiments of a receiver that employs afeedback circuit to improve the SNR of the sensed light signal.

FIG. 12 depicts an example process flow for an intelligently-controlledadaptive ladar receiver.

FIG. 13A depicts an example ladar receiver embodiment;

FIG. 13B depicts a plot of signal-to-noise ratio (SNR) versus range fordaytime use of the FIG. 13A ladar receiver embodiment as well asadditional receiver characteristics.

FIG. 14A depicts another example ladar receiver embodiment;

FIG. 14B depicts a plot of SNR versus range for daytime use of the FIG.14A ladar receiver embodiment as well as additional receivercharacteristics.

FIG. 15 depicts an example of motion-enhanced detector arrayexploitation.

FIG. 16 depicts plots showing motion-enhanced detector array trackingperformance.

FIG. 17 depicts different examples of pixel mask shapes that may beemployed when selecting pixel subsets for photodetector readout.

FIG. 18 depicts an example embodiment of a multi-channel ladar receiver.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1A illustrates an example embodiment of a ladartransmitter/receiver system 100. The system 100 includes a ladartransmitter 102 and a ladar receiver 104, each in communication withsystem interface and control 106. The ladar transmitter 102 isconfigured to transmit a plurality of ladar pulses 108 toward aplurality of range points 110 (for ease of illustration, a single suchrange point 108 is shown in FIG. 1A). Ladar receiver 104 receives areflection 112 of this ladar pulse from the range point 110. Ladarreceiver 104 is configured to receive and process the reflected ladarpulse 112 to support a determination of range point distance andintensity information. Example embodiments for innovative ladarreceivers 104 are described below.

In an example embodiment, the ladar transmitter 102 can take the form ofa ladar transmitter that includes scanning mirrors and uses a rangepoint down selection algorithm to support pre-scan compression (whichcan be referred herein to as “compressive sensing”), as shown by FIG.1B. However, different scanning arrangements and techniques can be usedby a practitioner if desired as there are numerous, and constantlyexpanding, methods of scanning that can be useful with ladar. Examplesinclude galvo-mirrors, micro-galvo mirrors, MEMS mirrors, liquid crystalscanning (e.g., waveguides, either reflective or transmissive), ormicrofluidic mirrors. Regardless of how the scanning is performed, suchan embodiment may also include an environmental sensing system 120 thatprovides environmental scene data to the ladar transmitter to supportthe range point down selection. Example embodiments of such ladartransmitter designs can be found in U.S. patent application Ser. No.62/038,065, filed Aug. 15, 2014 and U.S. Pat. App. Pubs. 2016/0047895,2016/0047896, 2016/0047897, 2016/0047898, 2016/0047899, 2016/0047903,and 2016/0047900, the entire disclosures of each of which areincorporated herein by reference. Through the use of pre-scancompression, such a ladar transmitter can better manage bandwidththrough intelligent range point target selection. FIG. 2 illustrates anexample block diagram for an example embodiment of a ladar receiver 104.The ladar receiver comprises detection optics 200 that receive lightthat includes the reflected ladar pulses 112. The detection optics 200are in optical communication with a light sensor 202, and the lightsensor 202 generates signals indicative of the sensed reflected ladarpulses 112. Signal read out circuitry 204 reads the signals generated bythe sensor 202 to generate signal data that is used for data creationwith respect to the range points (e.g., computing range point distanceinformation, range point intensity information, etc.). It should beunderstood that the ladar receiver 104 may include additional componentsnot shown by FIG. 2. FIGS. 3A-5B show various example embodiments ofdetection optics 200 that may be used with the ladar receiver 104. Thelight sensor 202 may comprise an array of multiple individuallyaddressable light sensors (e.g., an n-element photodetector array). Asan example embodiment, the light sensor 202 can take the form of a PINphotodiode array (e.g., an InGaAs PIN array). As another exampleembodiment, the light sensor 202 can take the form of a siliconavalanche photodiode (APD) array (e.g., an InGaAs APD array). Thereadout circuitry 204 can take any of a number of forms (e.g., a readout integrated circuit (ROTC)), and example embodiments for the readoutcircuitry are described below.

FIG. 3A illustrates an example embodiment of detection optics 200 for aladar receiver 104 which employs a non-imaging light collector 302.Thus, the non-imaging light collector 302 such as a compound parabolicconcentrator, does not re-image the image plane at its entrance fixedpupil 304 onto the light sensor 202 with which it is bonded at its exitaperture. With such an example embodiment, a lens 300 that includes animaging system for focusing light is in optical communication with thenon-imaging light collector 302. In the example of FIG. 3A, the lens 300is positioned and configured such that the lens focuses light (imageplane) at the entrance pupil 304 of the light collector 302 even thoughthere is no actual image at the bonded light sensor.

FIG. 3B illustrates another example embodiment of detection optics 200which employ a non-imaging light collector 302. With such an exampleembodiment, an afocal lens group 310 is in optical communication withthe non-imaging light collector 302. The light collector 302 includes anentrance pupil 304, and it can be bonded with the light sensor 202 atits exit aperture. In the example of FIG. 3B, the lens 310 is positionedand configured such that the entrance pupil of the afocal lens group isre-imaged at the entrance pupil 304 of the light collector 302. Theinventor also notes that if desired by a practitioner, the FIG. 3Bembodiment may omit the afocal lens 310.

With the example embodiments of FIGS. 3A and 3B, the light collector 302can take forms such as a fiber taper light collector or a compoundparabolic concentrator. An example fiber taper light collector isavailable from Schott, and an example compound parabolic concentrator isavailable from Edmunds Optics.

The example embodiments of FIGS. 3A and 3B provide various benefits topractitioners. For example, these example embodiments permit the use ofrelatively small detector arrays for light sensor 202. As anotherexample, these embodiments can be useful as they provide a practitionerwith an opportunity to trade detector acceptance angle for detector sizeas well as trade SNR for high misalignment tolerance. However, theembodiments of FIGS. 3A and 3B do not produce optimal SNRs relative toother embodiments.

FIG. 4 illustrates an example embodiment of detection optics 200 whichemploy an imaging light collector 320. Thus, the imaging light collector320 re-images the image received at its entrance pupil 304 onto thelight sensor 202. With such an example embodiment, a lens 300 thatincludes an imaging system for focusing light is in opticalcommunication with the imaging light collector 320. The lens ispositioned and configured such that the lens focuses light (image plane)at the entrance pupil 304 of the light collector 302, and the lightcollector 320 images this light onto the bonded light sensor 202. In anexample embodiment, the light collector 320 can take the form of acoherent fiber taper light collector. An example coherent fiber taperlight collector is available from Schott.

The example embodiment of FIG. 4 also provides various benefits topractitioners. For example, as with the examples of FIGS. 3A and 3B, theexample embodiment of FIG. 4 permits the use of relatively smalldetector arrays for light sensor 202. This embodiment can also be usefulfor providing a practitioner with an opportunity to trade detectoracceptance angle for detector size as well as trade SNR for highmisalignment tolerance. A benefit of the FIG. 4 example embodimentrelative to the FIGS. 3A/3B example embodiments is that the FIG. 4example embodiment generally produces higher SNR.

FIG. 5A illustrates an example embodiment of “direct to detector”detection optics 200 for a ladar receiver 104. With such an exampleembodiment, a lens 300 that includes an imaging system for focusinglight is in optical communication with the light sensor 202. The lens300 is positioned and configured such that the lens focuses light (imageplane) directly onto the light sensor 202. Thus, unlike the embodimentof FIGS. 3A and 4, there is no light collector between the lens 300 andthe light sensor 202.

FIG. 5B illustrates another example embodiment of “direct to detector”detection optics 200 for a ladar receiver 104. With such an exampleembodiment, an afocal lens 310 is in optical communication with thelight sensor 202. The lens 310 is positioned and configured such thatthe lens pupil is re-imaged directly onto the light sensor 202. Theinventor also notes that if desired by a practitioner, the FIG. 5Bembodiment may omit the afocal lens 310.

The example embodiments of FIGS. 5A and 5B are expected to require alarger detector array for the light sensor 202 (for a given system fieldof view (FOV) relative to other embodiments), but they are also expectedto exhibit very good SNR. As between the embodiments of FIGS. 5A and 5B,the embodiment of FIG. 5A will generally exhibit better SNR than theembodiment of FIG. 5B, but it is expected that the embodiment of FIG. 5Bwill generally be more tolerant to misalignment (which means the FIG. 5Bembodiment would be easier to manufacture). A purely non-imaging systemis expected to lead to a reduction in the range of the control optionsdiscussed below, since all pixels would ingest light from alldirections. However, the considerations of fault toleration and dynamicrange are expected to remain operable.

It should also be understood that the detection optics 200 can bedesigned to provide partial imaging of the image plane with respect tothe light sensor 202 if desired by a practitioner. While this wouldresult in a somewhat “blurry” image, such blurriness may be suitable fora number of applications and/or conditions involving low fill factordetector arrays. An example of such a partial imager would be acollection of compound parabolic concentrators, oriented in slightlydifferent look directions.

FIG. 6A illustrates an example embodiment for readout circuitry 204within a ladar receiver that employs a multiplexer 604 for selectingwhich sensors 602 within a detector array 600 are passed to a signalprocessing circuit 606. In an example embodiment, the sensors 602 maycomprise a photodetector coupled to a pre-amplifier. In an exampleembodiment, the photodetector could be a PIN photodiode and theassociated pre-amplifier could be a transimpedance amplifier (TIA). Inthe example embodiment depicted by FIG. 6A, the light sensor 202 takesthe forms of a detector array 600 comprising a plurality ofindividually-addressable light sensors 602. Each light sensor 602 can becharacterized as a pixel of the array 600, and each light sensor 602will generate its own sensor signal 610 in response to incident light.Thus, the array 600 can comprise a photodetector with a detection regionthat comprises a plurality of photodetector pixels. The embodiment ofFIG. 6A employs a multiplexer 604 that permits the readout circuitry 204to isolate the incoming sensor signals 610 that are passed to the signalprocessing circuit 606 at a given time. In doing so, the embodiment ofFIG. 6A provides better received SNR, especially against ambient passivelight, relative to ladar receiver designs such as those disclosed byU.S. Pat. No. 8,081,301 where no capability is disclosed for selectivelyisolating sensor readout. Thus, the signal processing circuit 606 canoperate on a single incoming sensor signal 610 (or some subset ofincoming sensor signals 610) at a time.

The multiplexer 604 can be any multiplexer chip or circuit that providesa switching rate sufficiently high to meet the needs of detecting thereflected ladar pulses. In an example embodiment, the multiplexer 604multiplexes photocurrent signals generated by the sensors 602 of thedetector array 600. However, it should be understood that otherembodiments may be employed where the multiplexer 604 multiplexes aresultant voltage signal generated by the sensors 602 of the detectorarray 600. Moreover, in example embodiments where a ladar receiver thatincludes the readout circuitry 204 of FIG. 6A is paired with a scanningladar transmitter that employs pre-scan compressive sensing (such as theexample embodiments employing range point down selection that aredescribed in the above-referenced and incorporated patent applications),the selective targeting of range points provided by the ladartransmitter pairs well with the selective readout provided by themultiplexer 604 so that the receiver can isolate detector readout topixels of interest in an effort to improve SNR.

A control circuit 608 can be configured to generate a control signal 612that governs which of the incoming sensor signals 610 are passed tosignal processing circuit 606. In an example embodiment where a ladarreceiver that includes the readout circuitry 204 of FIG. 6A is pairedwith a scanning ladar transmitter that employs pre-scan compressivesensing according to a scan pattern, the control signal 612 can causethe multiplexer to selectively connect to individual light sensors 602in a pattern that follows the transmitter's shot list (examples of theshot list that may be employed by such a transmitter are described inthe above-referenced and incorporated patent applications). The controlsignal 612 can select sensors 602 within array 600 in a pattern thatfollows the targeting of range points via the shot list. Thus, if thetransmitter is targeting pixel x,y in the scan area with a ladar pulse,the multiplexer 604 can generate a control signal 612 that causes areadout of pixel x,y from the detector array 600.

FIG. 8 shows an example embodiment for control circuit 608. The controlcircuit 608 receives the shot list 800 as an input. This shot list is anordering listing of the pixels within a frame that are to be targeted asrange points by the ladar transmitter. These shot list pixels can beidentified by their coordinates in a pixel coordinate system. At 802,the control circuit selects a first of the range points/target pixels onthe shot list. At 804, the control circuit maps the selected range pointto a sensor/pixel (or a composite pixel/superpixel) of the detectorarray 600. In an example embodiment, the coordinate system for thetargeted range point pixels from the shot list 800 is the same as thecoordinate system for the pixels of the array 600. In such a case, for ascenario where the receiver will receive from a single pixel (ratherthan a composite pixel/superpixel), the mapping step 804 is reduced tosimply selecting the targeted pixel from the shot list 800; and for ascenario where the receiver will receive from a compositepixel/superpixel, the mapping step 804 would involve selecting the setof receiver pixels that are members of the composite pixel/superpixelbased on the targeted pixel from the shot list (which may or may notinclude the targeted range point pixel, as indicated by the example ofFIG. 17 discussed below). In another example embodiment where the rangepoint pixels and the receiver pixels do not share the same coordinatesystem, the mapping step 804 can also involve translating the rangepoint pixel coordinates from the coordinate system of the shot listpixels to the coordinate system of the receiver pixels. At 806, thecontrol circuit then generates a control signal 612 that is effective tocause the multiplexer to readout the mapped sensor/pixel (or compositepixel/superpixel) of the detector array 600. At 808, the control circuitprogresses to the next range point/target pixel on the shot list andreturns to operation 802. If necessary, the control circuit 608 caninclude timing gates to account for round trip time with respect to theladar pulses targeting each pixel.

The amplifiers integrated into the sensors 602 to providepre-amplification can be configured to provide a high frequency 3 dBbandwidth on the order of hundreds of MHz, e.g, around 100 MHz to around1000 MHz, in order to provide the requisite bandwidth to resolvedistances on the order of a few cm, e.g., around 1 cm to around 10 cm.The inventors further observe that an array with hundreds or thousands(or even more) wide bandwidth amplifiers may present a power budget thatcould adversely affect operations in certain circumstances. Furthermore,the heat associated with a large number of closely-spaced amplifiers maylead to non-trivial challenges associated with thermal dissipation.However, the ability to adaptively select which subsets of pixels willbe read out at any given time provides an elegant manner of solvingthese problems. For example, by using the a priori knowledge from theshot list (which defines the sequence in which the pixels (and compositepixels) will be selected for readout), the system can avoid the need toprovide full power to all of the amplifiers at the same time (and avoidproducing the heat that would be attendant to powering such amplifiers).

It should be understood that the control signal 612 can be effective toselect a single sensor 602 at a time or it can be effective to selectmultiple sensors 602 at a time in which case the multiplexer 604 wouldselect a subset of the incoming sensor signals 610 for furtherprocessing by the signal processing circuit 606. Such multiple sensorscan be referred to as composite pixels (or superpixels). For example,the array 600 may be divided into a J×K grid of composite pixels, whereeach composite pixel is comprised of X individual sensors 602. Summercircuits can be positioned between the detector array 600 and themultiplexer 604, where each summer circuit corresponds to a singlecomposite pixel and is configured to sum the readouts (sensor signals610) from the pixels that make up that corresponding composite pixel.

It should also be understood that a practitioner may choose to includesome pre-amplification circuitry between the detector array 600 and themultiplexer 604 if desired.

FIG. 6B depicts an example ladar receiving method corresponding to theexample embodiment of FIG. 6A. At step 620, a ladar pulse is transmittedtoward a targeted range point. As indicated above, a location of thistargeted range point in a scan area of the field of view can be known bythe ladar transmitter. This location can be passed from the ladartransmitter to the ladar receiver or determined by the ladar receiveritself, as explained below.

At step 622, a subset of pixels in the detector array 600 are selectedbased on the location of the targeted range point. As indicated inconnection with FIG. 8, a mapping relationship can be made betweenpixels of the detector array 600 and locations in the scan area suchthat if pixel x1,y1 in the scan area is targeted, this can be translatedto pixel j1,k1 in the detector array 600. It should be understood thatthe subset may include only a single pixel of the detector array 600,but in many cases the subset will comprise a plurality of pixels (e.g.,the specific pixel that the targeted range point maps to plus somenumber of pixels that surround that specific pixel). Such surroundingpixels can be expected to also receive energy from the range point ladarpulse reflection, albeit where this energy is expected to be lower thanthe specific pixel.

At step 624, the selected subset of pixels in the detector array 600senses incident light, which is expected to include thereflection/return of the ladar pulse transmitted at step 620. Each pixelincluded in the selected subset will thus produce a signal as a functionof the incident sensed light (step 626). If multiple pixels are includedin the selected subset, these produced pixel-specific signals can becombined into an aggregated signal that is a function of the incidentsensed light on all of the pixels of the selected subset. It should beunderstood that the detector pixels that are not included in theselected subset can also produce an output signal indicative of thelight sensed by such pixels, but the system will not use these signalsat steps 626-630. Furthermore, it should be understood that the systemcan be configured to “zero out” the pixels in the selected subset priorto read out at steps 624 and 626 eliminate the effects of anystray/pre-existing light that may already be present on such pixels.

At step 628, the photodetector signal generated at step 626 isprocessed. As examples, the photodetector signal can be amplified anddigitized to enable further processing operations geared towardresolving range and intensity information based on the reflected ladarpulse. Examples of such processing operations are discussed furtherbelow.

At step 630, range information for the targeted range point is computedbased on the processing of the photodetector signal at step 628. Thisrange computation can rely on any of a number of techniques. Also, thecomputed range information can be any data indicative of a distancebetween the ladar system 100 and the targeted range point 110. Forexample, the computed range information can be an estimation of the timeof transit for the ladar pulse 108 from the transmitter 102 to thetargeted range point 110 and for the reflected ladar pulse 112 from thetargeted range point 110 back to the receiver 104. Such transit timeinformation is indicative of the distance between the ladar system 100and the targeted range point 110. For example, the range computation canrely on a measurement of a time delay between when the ladar pulse wastransmitted and when the reflected ladar pulse was detected in thesignal processed at step 628. Examples of techniques for supporting suchrange computations are discussed below.

It should be understood that the process flow of FIG. 6B describes anadaptive ladar receiving method where the active sensing region of thedetector array 600 will change based on where the ladar pulses aretargeted by the ladar transmitter. In doing so, it is believed thatsignificant reductions in noise and improvements in range resolutionwill be achieved. Further still, as explained in greater detail below,the subset of detector pixels can be adaptively selected based oninformation derived from the sensed light to further improveperformance.

Returning to FIG. 6A, the signal processing circuit 606 can beconfigured to amplify the selected sensor signal(s) passed by themultiplexer 604 and convert the amplified signal into processed signaldata indicative of range information and/or intensity for the ladarrange points. Example embodiments for the signal processing circuit 606are shown by FIGS. 7A and 7B.

In the example of FIG. 7A, the signal processing circuit 606 comprisesan amplifier 700 that amplifies the selected sensor signal(s), ananalog-to-digital converter (ADC) 702 that converts the amplified signalinto a plurality of digital samples, and a field programmable gate array(FPGA) that is configured to perform a number of processing operationson the digital samples to generate the processed signal data. It shouldbe understood that the signal processing circuit 606 need notnecessarily include an FPGA; the processing capabilities of the signalprocessing circuit 606 can be deployed in any processor suitable forperforming the operations described herein, such as a central processingunit (CPU), micro-controller unit (MCU), graphics processing unit (GPU),digital signal processor (DSP), and/or application-specific integratedcircuit (ASIC) or the like.

The amplifier 700 can take the form of a low noise amplifier such as alow noise RF amplifier or a low noise operational amplifier. The ADC 702can take the form of an N-channel ADC.

The FPGA 704 includes hardware logic that is configured to process thedigital samples and ultimately return information about range and/orintensity with respect to the range points based on the reflected ladarpulses. In an example embodiment, the FPGA 704 can be configured toperform peak detection on the digital samples produced by the ADC 702.In an example embodiment, such peak detection can be effective tocompute range information within +/−10 cm. The FPGA 704 can also beconfigured to perform interpolation on the digital samples where thesamples are curve fit onto a polynomial to support an interpolation thatmore precisely identifies where the detected peaks fit on the curve. Inan example embodiment, such interpolation can be effective to computerange information within +/−5 mm.

When a receiver which employs a signal processing circuit such as thatshown by FIG. 7A is paired with a ladar transmitter that employscompressive sensing as described in the above-referenced andincorporated patent applications, the receiver will have more time toperform signal processing on detected pulses because the ladartransmitter would put fewer ladar pulses in the air per frame than wouldconventional transmitters, which reduces the processing burden placed onthe signal processing circuit. Moreover, to further improve processingperformance, the FPGA 704 can be designed to leverage the parallelhardware logic resources of the FPGA such that different parts of thedetected signal are processed by different hardware logic resources ofthe FPGA at the same time, thereby further reducing the time needed tocompute accurate range and/or intensity information for each rangepoint.

Furthermore, the signal processing circuit of FIG. 7A is capable ofworking with incoming signals that exhibit a low SNR due to the signalprocessing that the FPGA can bring to bear on the signal data in orderto maximize detection. The SNR can be further enhanced by varying thepulse duration on transmit. For example, if the signal processingcircuit reveals higher than usual clutter (or the presence of otherlaser interferers) at a range point, this information can be fed back tothe transmitter for the next time that the transmitter inspects thatrange point. A pulse with constant peak power but extended by a multipleof G will have G times more energy. Simultaneously, it will possess Gtimes less bandwidth. Hence, if we low pass filter digitally, the SNR isexpected to increase by G^(3/2), and the detection range for fixedreflectivity is expected to increase by G^(3/4). This improvement isexpected to hold true for all target-external noise sources: thermalcurrent noise (also called Johnson noise), dark current, and background,since they all vary as √{square root over (G)}. The above discussionentails a broadened transmission pulse. Pulses can at times be stretcheddue to environmental effects. For example, a target that has a projectedrange extent within the beam diffraction limit will stretch the returnpulse. Digital low pass filtering is expected to improve the SNR here by√{square root over (G)} without modifying the transmit pulse. The pulseduration can also be shortened, without a loss in SNR on transmit, inorder to reduce pulse stretching from the environment. Shortening alsoenables a reduction in pulse energy, as desired at short range. Theabove analysis assumes white noise, but the practitioner will recognizethat extensions to other noise spectrum are straightforward.

In the example of FIG. 7B, the signal processing circuit 606 comprisesthe amplifier 700 that amplifies the selected sensor signal(s) and atime-to-digital converter (TDC) 710 that converts the amplified signalinto a plurality of digital samples that represent the sensed light(including reflected ladar pulses). The TDC can use a peak and holdcircuit to detect when a peak in the detected signal arrives and alsouse a ramp circuit as a timer in conjunction with the peak and holdcircuit. The output of the TDC 710 can then be a series of bits thatexpresses timing between peaks which can be used to define rangeinformation for the range points.

The signal processing circuit of FIG. 7B generally requires that theincoming signals exhibit a higher SNR than the embodiment of FIG. 7A,but the signal processing circuit of FIG. 7B is capable of providinghigh resolution on the range (e.g., picosecond resolution), and benefitsfrom being less expensive to implement than the FIG. 7A embodiment.

FIG. 9 discloses an example embodiment where the ladar transmitter 102and a photodetector 900 are used to provide the ladar receiver 104 withtracking information regarding where the ladar transmitter (via itsscanning mirrors) is targeted. In this example, photodetector 900 ispositioned optically downstream from the scanning mirrors (e.g., at theoutput from the ladar transmitter 102), where this photodetector 900operates as (1) an effectively transparent window for incident lightthat exhibits a frequency within a range that encompasses thefrequencies that will be exhibited by the ladar pulses 108 (where thisfrequency range can be referred to as a transparency frequency range),and (2) a photodetector for incident light that exhibits a frequencythat is not within the transparency frequency range. Thus, thedoped/intrinsic layer and the substrate of the photodetector can bechosen so that the ladar pulses 108 fall within the transparencyfrequency range while light at another frequency is absorbed anddetected. The region of the photodetector that exhibits this dualproperty of transmissiveness versus absorption/detection based onincident light frequency can be housed in an opticallytransparent/transmissive casing. The electronic circuitry ofphotodetector 900 that supports the photodetection operations can behoused in another region of the photodetector 900 that need not betransparent/transmissive. Such a photodetector 900 can be referred to asa dichroic photodetector.

The ladar transmitter 102 of FIG. 9 is equipped with a second lightsource (e.g., a second bore-sighted light source) that outputs light 902at a frequency which will be absorbed by the photodetector 900 andconverted into a photodetector output signal 904 (e.g., photocurrent q).Light 902 can be laser light, LED light, or any other light suitable forprecise localized detection by the photodetector 900. The ladartransmitter 102 can align light 902 with ladar pulse 108 so that thescanning mirrors will direct light 902 in the same manner as ladar pulse108. The photodetector's output signal 904 will be indicative of the x,yposition of where light 902 strikes the photodetector 900. Due to thealignment of light 902 with ladar pulse 108, this means that signal 904will also be indicative of where ladar pulse 108 struck (and passedthrough) the photodetector 900. Accordingly, signal 904 serves as atracking signal that tracks where the ladar transmitter is targeted asthe transmitter's mirrors scan. With knowledge of when each ladar pulsewas fired by transmitter 102, tracking signal 904 can thus be used todetermine where the ladar transmitter was aiming when a ladar pulse 108is fired toward a range point 110. We discuss below how timing knowledgeabout this firing can be achieved. Tracking signal 904 can then beprocessed by a control circuit in the ladar receiver 104 or otherintelligence within the system to track where ladar transmitter 102 wastargeted when the ladar pulses 108 were fired. By knowing preciselywhere the transmitter is targeted, the system is able to get improvedposition location of the data that is collected by the receiver. Theinventors anticipate that the system can achieve 1 mrad or better beampointing precision for a beam divergence of around 10 mrad. This allowsfor subsequent processing to obtain position information on the rangepoint return well in excess of the raw optical diffraction limit.

In the example embodiment of FIG. 9, we have chosen to have thecalibration light 902 absorbed by the photodetector, whilst theenvironmental probe laser is transmitted through the detector unabated(pulses 108). In order to enhance the transmission of the environmentalprobe laser (pulses 108) through a high index photodetector, ananti-reflection coating on the photodetector 900 can be used to enhanceperformance, where this anti-reflection coating is optimized for thewavelength of the ladar pulses 108 on the front and rear surfaces of thephotodetector. This embodiment enables two normally separate systems (aphotodetector and beam splitter) to be merged into one. It is alsopossible to coat the surface of the photodetector that faces theincoming laser with a dichroic material, now tuned to the high-powerlaser wavelength, to reflect the environmental probe laser while thecalibration laser passes through the coating and is detected by thephotodetector. This then servers the purpose of merging both thephotodetector and the beam splitter and no longer requires that thedichroic mirror be transparent to the environmental probe laser(although it does involve re-directing the high-powered laser).

We will now discuss time of transmit and time of receipt for laserlight. FIG. 10A discloses an example embodiment where an optical pathdistinct from the path taken by ladar pulse 108 from the transmitter 102toward a range point and back to the receiver 104 via ladar pulsereflection 112 is provided between the ladar transmitter 102 and ladarreceiver 104, through which reference light 1000 is communicated fromtransmitter 102 to receiver 104, in order to improve range accuracy.Furthermore, this distinct optical path is sufficient to ensure that thephotodetector 600 receives a clean copy of the reference light 1000.

This distinct optical path can be a direct optical path from thetransmitter 102 to the receiver's photodetector 600. With such a directoptical path, the extra costs associated with mirrors or fiber optics toroute the reference light 1000 to the receiver's photodetector 600 canbe avoided. For example, in an arrangement where the transmitter andreceiver are in a side-by-side spatial arrangement, the receiver 104 caninclude a pinhole or the like that passes light from the transmitter 102to the photodetector 600. In practice this direct optical path can bereadily assured because the laser transmit power is considerablystronger than the received laser return signal. For instance, at 1 km,with a 1 cm receive pupil, and 10% reflectivity, the reflected lightsensed by the receiver will be over 1 billion times smaller than thelight at the transmitter output. Hence a small, um scale, pinhole in theladar receiver casing at 104, with the casing positioned downstream fromthe output of mirror 904 would suffice to establish this direct link. Inanother embodiment, a fiber optic feed can be split from the main fiberlaser source and provide the direct optical path used to guide thereference light 1000, undistorted, onto the photodetector.

The reference light 1000, spawned at the exact time and exact locationas the ladar pulse 108 fired into the environment, can be the same pulseas ladar pulse 108 to facilitate time delay measurements for use inrange determinations. In other words, the reference light 1000 comprisesphotons with the same pulse shape as those sent into the field. However,unlike the ladar pulse reflection from the field, the reference lightpulse is clean with no noise and no spreading.

Thus, as shown in the example expanded view of the ladar receiver 104 inFIG. 10A, the photodetector 600 receives the reference pulse 1000 viathe distinct optical path and then later the reflected ladar pulse 112.The signal sensed by the photodetector 600 can then be digitized by anADC 1002 and separated into two channels. In a first channel, a delaycircuit/operator 1004 delays the digitized signal 1006 to produce adelayed signal 1008. The delayed signal 1008 is then compared with thedigitized signal 1006 via a correlation operation 1010. This correlationoperation can be the multiplication of each term 1006, 1008 summedacross a time interval equal to or exceeding the (known) pulse length.As signal 1006 effectively slides across signal 1008 via the correlationoperation 1010, the correlation output 1012 will reach a maximum valuewhen the two signals are aligned with each other. This alignment willindicate the delay between reference pulse 1000 and reflected pulse 112,and this delay can be used for high resolution range determination. Forexample, suppose, the reference light signal 1000 arrives 3 digitalsamples sooner than the reflected ladar pulse 112. Assume these twosignals are identical (no pulse spreading in the reflection), and equal,within a scale factor, {1,2,1}, i.e. the transmit pulse lasts threesamples. Then for a delay of zero in 1004, summing twice the pulselength, the output is {1,2,1,0,0,0} times {0,0,0,1,2,1}. Next suppose wedelay by 1 sample in 1004. Then the output is sum[{0,1,2,1,0,0} times{0,0,0,1,2,1}]=1. If we increment the delay by 1 sample again, we get 4as the correlation output 1012. For the next sample delay increment, weget a correlation output of 6. Then, for the next sample delayincrement, we get a correlation output of 4. For the next two sampledelay increments we get correlation outputs of 1 and then zerorespectively. The third sample delay produces the largest correlationoutput, correctly finding the delay between the reference light and thereflected ladar pulse. Furthermore, given that for a range of 1 km, thetransmitter can be expected to be capable of firing 150,000 pulses everysecond, it is expected that there will be sufficient timing space forensuring that the receiver gets a clean copy of the reference light 1000with no light coming back from the ladar pulse reflection 112. The delayand correlation circuit shown by FIG. 10A can also be referred to as amatched filter. The matched filter can be implemented in an FPGA orother processor that forms part of signal processing circuit 606.

While the example of FIG. 10A shows a single photodetector 600 and ADC1002 in the receiver, it should be understood that separatephotodetectors can be used to detect the return pulse 112 and thereference pulse 1000. Also, separate ADCs could be used to digitize theoutputs from these photodetectors. We can also use separatephotodetectors for the reference and return pulses and sum the analogsignals prior to a single ADC. However, it is believed that the use of asingle photodetector and ADC shared by the return pulse 112 andreference pulse 114 will yield to cost savings in implementation withoutloss of performance. Also, interpolation of the sampled return pulse 112can be performed as well using pulse 1000 as a reference. After peakfinding, conducted using the process described above, the system canfirst interpolate the reference light signal. This can be done using anydesired interpolation scheme, such as cubic spline, sinc functioninterpolation, zero pad and FFT, etc. The system then interpolates thereceive signal around the peak value and repeats the process describedabove. The new peak is now the interpolated value. Returning to ourprevious example, suppose we interpolate the reference light pulse toget {1,1.5,2,1.5,1,0,0,0,0,0,0}, and we interpolate the receive pulselikewise to get {0,0,0,1,1,5,2,1,5,1}. Then the system slides,multiplies, and sums. The advantage of this, over simply “trusting” theladar return interpolation alone, is that the correlation with thereference light removes noise from the ladar return.

Making reference pulse 1000 the same as ladar pulse 108 in terms ofshape contributes to the improved accuracy in range detection becausethis arrangement is able to account for the variation in pulse 108 fromshot to shot. Specifically, range is improved from the shape, andreflectivity measurement is improved by intensity, using pulse energycalibration (which is a technique that simply measures energy ontransmit). The range case is revealed in modeling results shown by FIG.10B. The vertical axis of FIG. 10B is range accuracy, measured as ±x cm,i.e. x standard deviations measured in cm, and the horizontal axis ofFIG. 10B is the SNR. This model is applied to a 1 ns full width halfmaximum Gaussian pulse. The bottom line plotted in FIG. 10B is the idealcase, The nearby solid line 121 is the plot for an ADC with 1 picosecondof timing jitter, which is a jitter level readily available commerciallyfor 2 GHz ADCs. By comparing the performance of the two curves indicatedbelow 121, one can see from FIG. 10B that jitter is not a limitingfactor in achieving sub-cm resolution. Specifically the lower curve (nojitter) and upper curve (jitter) differ by only a millimeter at veryhigh (and usually unachievable) SNR [˜1000]. However, pulse variation isa significant limitation. This is seen by 120, which is the performanceavailable with 5% pulse-to-pulse shape variation, a common limit incommercial nanosecond-pulsed ladar systems. The difference between 120and 121 is the improvement achieved by example embodiments of thedisclosed FIG. 10A technique, for both peak finding and interpolation asa function of SNR.

We conclude the discussion of range precision by noting that thecomputational complexity of this procedure is well within the scope ofexisting FPGA devices. In one embodiment, the correlation andinterpolation can be implemented after a prior threshold is crossed bythe data arriving from the reflected lidar pulse. This greatly reducescomplexity, at no performance cost. Recall, the intent of correlationand interpolation is to improve ranging—not detection itself, sodelaying these operations and applying them only around neighborhoods ofdetected range returns streamlines computations without erodingperformance. Typically only 3 samples are taken of the reference lightpulse since it is so short. Interpolating this 20-fold using cubicmodels requires only about 200 operations, and is done once per shot,with nominally 100,000 shots. The total burden pre matching filter andinterpolation against the ladar receive pulse is then 20Mflops. If weselect the largest, first and last pulse for processing, this rises toless than 100Mflop, compared to teraflops available in modern commercialdevices.

Furthermore, FIG. 11A discloses an example embodiment of a receiverdesign that employs a feedback circuit 1100 to improve the SNR of thesignals sensed by the active sensors/pixels 602. The feedback circuit1100 can be configured as a matching network, in resonance with thereceived ladar pulse return 112 (where the ladar pulse 108 and returnpulse 112 can exhibit a Gaussian pulse shape in an example embodiment),thereby enhancing the signal and retarding the noise. A photodetectorperformance is a function of pitch (area of each element) and bandwidth.Passive imagers lack prior knowledge of incident temporal signalstructure and have thus no ability to tune performance. However, inexample embodiments where the ladar transmitter employs compressivesensing, the transmitted ladar pulse 108 is known, as it is arrival timewithin the designated range swath. This knowledge can facilitate amatching network feedback loop that filters the detector current,increases signal strength, and filters receiver noise. A feedback gainprovided by the feedback circuit can be controlled via a control signal1102 from control circuit. Furthermore, it should be understood that thecontrol circuit 608 can also be in communication with the signalprocessing circuit 606 in order to gain more information about operatingstatus for the receiver.

The matching network of the feedback circuit 1100 may be embedded intothe In-GaAs substrate of detector 600 to minimize RF coupling noise andcross channel impedance noise. The cost of adding matching networks ontothe detector chip is minimal. Further, this matching allows us to obtainbetter dark current, ambient light, and Johnson noise suppression thanis ordinarily available. This further reduces required laser power,which, when combined with a 1.5 um wavelength for ladar pulses 108leads, to a very eye safe solution. The matching network can becomprised of more complex matching networks with multiple poles,amplifiers, and stages. However, a single pole already providessignificant benefits. Note that the input to the signal processingcircuit 606 can be Gaussian, regardless of the complexity of themultiplexer, the feedback, or the size variability of the pixels, due tothe convolutional and multiplicative invariance of this kernel.

FIG. 11B shows an example that expands on how the feedback circuit 1100can be designed. The matching network involves one or more amplifiers1102, in a controlled feedback loop 1104 with a gain controllerfurnished by the control circuit 608. The matching network can bepresent on all the input lines to the mux 604 (e.g., there can be oneamplifier in the photodetector per pixel), and FIG. 11B shows just asingle such network, within the dotted box 1120, for ease ofillustration. The feedback gain is generally chosen to output maximalSNR using differential equations to model the input/output relationshipsof the feedback circuit. In practice the control loop can be designed tomonitor the mux output and adjust the amplifiers 1102 to account fromdrift due to age, thermal effects, and possible fluctuations in ambientlight. Although also disclosed herein are embodiments which employ twoor more digital channels to build a filter (e.g. a Weiner filter orleast mean squares filter) to reject interference from strongscatterers, other ladar pulses, or even in-band sunlight, headlights orother contaminants. The two channels can, in an example embodiment, beset with different gain settings to simultaneously collect on strong andweak signals. Also, the feedback circuit can be reset at each shot toavoid any saturation from contamination in the output from shot to shot.

Feedback control can be simplified if a Gaussian pulse shape is used forladar pulse 108 in which case all the space time signals remain normallydistributed, using the notation in 1122. Accordingly, in an exampleembodiment, the ladar pulse 108 and its return pulse 112 can exhibit aGaussian pulse shape. In such an example embodiment (where the laserpulse 108 is Gaussian), the Fourier representation of the pulse is alsoGaussian, and the gain selection by the control circuit 608 istractable, ensuring rapid and precise adaptation.

Another innovative aspect of the design shown by FIG. 11B is the use ofhexagonally shaped pixels for a plurality of the sensors 602 within thephotodetector array 600, The shaded area 1130 indicates the selectedsubset of pixels chosen to pass to the signal processing circuit 606 ata given time. By adaptively selecting which pixels 602 are selected bythe multiplexer 604, the receiver can grow or shrink the size of theshaded area 1130, either by adding or subtracting pixels/sensors 602.The hexagonal shape of pixels/sensors 602 provides a favorable shape forfault tolerance since each hexagon has 6 neighbors. Furthermore, thepixels/sensors 602 of the photodetector array 600 can exhibit differentsizes and/or shapes if desired by a practitioner. For example, some ofthe pixels/sensors can be smaller in size (see 1132 for example) whileother pixels/sensors can be larger in size (see 1134). Furthermore, somepixels/sensors can be hexagonal, while other pixels/sensors can exhibitdifferent shapes (e.g., rectangular). While the example embodimentsdiscussed below are focused on an advanced receiver that operates inisolation, it should be understood that the receiver might be combinedwith other systems, such as scanning receive mirrors, or transmissiveequivalents, which might reduce required pixel count.

FIG. 11C shows another example embodiment that expands on how thefeedback matching network can be used in combination with shotpipelining to improve performance. Pipelining how the shots from theshot list 800 are processed within the control circuit 608 allows thesystem to fully power the amplifiers within the feedback matchingnetwork only when necessary. The amplifiers in the feedback matchingnetwork that correspond to pixels not needed for readout with respect toa targeted range point can be kept in a quiescent state (e.g., a lowerpower standby mode) in order to reduce power consumption. However, thisfeature in turn raises a problem arising from the settling time that isneeded by an amplifier for stable operation when powered up. Thepipelining feature disclosed herein provides an elegant solution to theconflicting concerns of reduced power consumption versus the need forsettling time when transitioning an amplifier from a quiescent state toa fully powered state. In an example embodiment of this concept, if thei^(th) composite pixel in the shot list is to measure the return fromthe laser pulse associated with the i^(th) laser trigger, the system canbe configured to power the i^(th) composite pixel at the time of the(i−1)^(th) laser trigger. This pixel is then ready to measure the returnassociated with the i^(th) laser trigger. At the time of the (i+1)^(th)laser trigger, the i^(th) composite pixel returns to the quiescentstate. If additional time is required to fully activate a pixel, thepipelining can be adjusted accordingly. Pipelining the shot list alsoreduces latency and increases speed.

To achieve pipelining, the process flow of FIG. 8 can be performed overa small, fixed, or variable-sized window of future shot list values.This will yield a precomputed set of values for a control loop that canbe pipelined through the system during operation and allows the controlcircuit 608 to turn “on” (i.e., power up from a quiescent state) theamplifiers for pixels shortly before they are needed and giving themtime to settle before readout occurs. As shown by FIG. 11C, as part ofthis pipelining, a delay 1116 can be injected as between the amplifiercontrol and the mux control.

FIG. 11C also shows an example of feedback circuit 1120 in greaterdetail, where the feedback circuit 1120 includes one or morecontrollable amplifiers 1102 in a feedback loop 1104 with a gaincontroller furnished by the control circuit 608. To keep the amplifiersin a quiescent state, a steady low current “trickle charge” can be fedto the amplifiers 1102 to keep them in a low power standby mode. As withFIG. 11B, the matching network can be present on all the photodetectors(one per pixel) and the amplifier output 1112 is addressable by themultiplexer 604. In this example, the amplifier 1102 is a transimpedanceamplifier with polarity reversal, but it should be understood that otheramplifiers could be employed. For example, any amplifier that has theeffective input/output action of a transimpedance gain, such as aresistor followed by a voltage amplifier or an integrator followed by adifferentiator could be employed. As with the FIG. 11B embodiment, thefeedback gain for amplifier 1102 can be chosen to output maximal SNRusing differential equations to model the input/output relationships ofthe feedback circuit. The input to the feedback loop is thephotodetector output current 1110, and the output 1112 is the mux inputfeed. The feedback loop can include a resistor 1114 that may be fixed orvariable. When powering up an amplifier 1102. the control circuit 608can increase the voltage provided to that amplifier 1102 via a gaincontrol signal. The resistor 1114 can be varied, for example, to tunethe bandwidth, for fixed capacitance.

FIGS. 11D and 11E further illustrate an example of how the pipeliningfeature can be implemented to control amplifier gains and muxselections. For the ease of illustration, FIGS. 11D and 11E are drawnwith reference to a 4-pixel array 600. It should be understood that inpractice, the array 600 will likely include significantly more pixels,but for the purpose of explanation the 4-pixel array is useful. Thecontrol circuit 608 can include a data shift register 1170 through whichshots from the shot list 800 are streamed. The data shift register 1170as shown in FIG. 11D includes 4 cells—shown as cells 1172, 1174, 1176,and 1178 in FIG. 11D (where cell 1172 serves as the first cell in theshift register 1170 and cell 1178 serves as the last cell in the shiftregister). It should be understood that the shift register may includemore cells if needed or desired by a practitioner.

At time t=0, the first shot on the shot list 800 can enter the firstcell 1172 of the data shift register 1170 (which would be a shot thatmaps to Pixel 1 (P1) of the photodetector 600). Then, at time t=1, P1 isshifted to the second cell 1174 of the data shift register 1170 and theshot that maps to Pixel 2 (P2) enters the first cell 1172 of the shiftregister 1170, and so on for subsequent shots on the shot list 800. FIG.11D shows a case corresponding to t=3 where P1 has shifted to the fourthcell 1178 of the data shift register while the shot that maps to Pixel 3(P3) has entered the first cell 1172 of the data shift register 1170.

Feedback amplifier control logic 1150 can tap into one or more cells ofthe data shift register to control how the feedback amplifiers arepowered up or powered down. For example, the feedback amplifier controllogic 1150 can tap into cell 1174 to facilitate a determination as towhich feedback amplifier(s) should be powered up, and it can tap intocell 1178 to facilitate a determination as to which feedbackamplifier(s) should be powered down. In this example, cell 1172 can becharacterized as an “amplifier power up” cell and cell 1178 can becharacterized as an “amplifier power down” cell. Meanwhile MUX selectioncontrol logic 1152 can tap into a cell that is downstream from theamplifier power up cell and upstream from the amplifier power down cellto control which input line(s) to the multiplexer will be selected forreadout. In this example, the MUX selection control logic 1152 taps intocell 1176, and this cell can be characterized as a “collection” cell.The spacing between these tapped cells and the shift rate of pixelsthrough the data shift register 1170 will define the time delay betweenamplifier selection and MUX selection with respect to pixels. Thus, itis desirable for a practitioner to space these tapped cells in a mannerthat accommodates the settling time needed for amplifiers 1102 whenpowering up from a quiescent state. In this example, this spacing can beequal to the time of flight of the shot so a single pipeline for both1150 and 1152 can be used. However, if necessary, a practitioner mightfind it appropriate to maintain multiple data shift pipelines (e.g., onedata shift register for controlling amplifier power up/power down andone data shift register for controlling MUX readout) with differentshift rates for the pipelines to achieve a desired timing betweenamplifier power up, amplifier settling, and MUX collection/readout.

Feedback amplifier control logic 1150 can execute a process flow similarin nature to the process flow of FIG. 8 where the logic 1150 maps thedata in cells 1174 and 1178 (where such data can be identifier(s) forone or more pixels) to a decision about an amplifier network controlsignal that is effective to power up the amplifier(s) corresponding tothe subject pixel(s) for the data in cell 1174 and power down theamplifier(s) corresponding to the subject pixel(s) for the data in cell1178. In the example of FIG. 11D, the amplifier network control signalcan include a signal 1154 provided to the amplifier (A1) for Pixel 1, asignal 1156 provided to the amplifier (A2) for Pixel 2, a signal 1158provided to the amplifier (A3) for Pixel 3), and a signal 1160 providedto the amplifier (A4) for Pixel 4. The value of these signals can governwhether their corresponding amplifiers are powered up or powered down.With P4 being present in cell 1174, this means that the feedbackamplifier control logic will drive signal 1160 in a manner that causesA4 to power up (or remain powered up as the case may be), as indicatedby the alternating dash/dot line for 1160. With P1 being present in cell1178, this means that the feedback amplifier control logic will drivesignal 1154 in a manner that causes A1 to power down (presuming it isnot otherwise needed because of a repeat visit to P1), as indicated bythe dashed line for 1154.

MUX selection control logic 1152 can execute a process flow of FIG. 8where the logic 1152 maps the data in cell 1176 (where such data can beidentifier(s) such as coordinates for one or more pixels) to a MUXcontrol signal 612 that is effective to cause the MUX to read out theamplified output from the subject pixel(s). In the example of FIG. 11D,with P1 being present in cell 1176, this means that the MUX controlsignal 612 will be effective to pass the input on M1 to the MUX output(as indicated by the boldface for the M1 label and dashed box around theboldfaced M1 label).

FIG. 11E shows a visual progression over time of pixel datacorresponding to shots on the shot list 800 through the data shiftregister 1170 in combination with a table 1180 that identifies thecorresponding control signals for amplifier and MUX selections. In theseexamples, the value “1” in the table would correspond to a “powered”state for the subject amplifiers (e.g., an awaken command) andcorrespond to a “select/pass” state for an input line to the MUX. Thevalue of “−1” for the amplifier control signals would correspond to a“power down” (or “sleep”) command that returns the subject amplifier toa quiescent state. It can be seen that it is desirable for the controllogic 1150 to keep a selected amplifier powered for a time durationsufficient to allow the amplifier to power up and settle prior to readout by the MUX. In this example, we use one shift register cycle as thattime interval, but it should be understood that other time intervalsmight be appropriate. After the MUX readout occurs, the subjectamplifier can be returned to the quiescent state (via the “−1” signal)presuming the shot list does not return to the pixel within the delaywindow 1116. For example, at t=t5, it can be seen that pixel P4 ispresent in both cells 1174 and 1178. In such a situation, the feedbackamplifier control logic 1150 can conclude that amplifier A4 for P4remain powered. As another example, it can be seen that at time t=t6,the amplifier A3 for pixel P3 is powered down although it needs to bepowered again for the next shift cycle (a similar state of affairsexists for A4 at time t=t7). It should be understood that a practitionermight find it more efficient and/or more effective to keep A3 powered att=t6 rather than powering it down. To support such decision-making, thefeedback amplifier control logic 1150 can also tap into the one or morebuffer cells 1170 that allow the system to track upcoming pixels. It canalso be seen that the amplifier control signals 1154-1160 will typicallyoperate such that multiple amplifiers are powered up at a given time anddifferent amplifiers return to a quiescent state at different times.Furthermore, it can be seen that cell 1172 serves as a buffer thatpositions the pixel indices so they are available to the control logicfor the next cycle.

It should also be understood that the examples of FIGS. 11D and 11E aresimple examples where each range point on shot list 800 is mapped to asingle pixel (presented in this fashion for ease of illustration). Inmany instances, the range points on the shot list 800 will map tomultiple pixels (for example, see the example superpixel shapes 1704,1706, 1708, 1710, and 1712 shown by FIG. 17 and discussed below).Accordingly, it should be understood that the feedback amplifier controllogic 1150 can operate to power up and power down multiple amplifierseach cycle and the MUX selection control logic 1152 can operate toselect multiple input lines each cycle. When a targeted range pointpixel maps to a composite pixel/superpixel in the receiver, thisone-to-many mapping can be performed by control logic 1150 and 1152positioned as shown by FIG. 11D (where each of 1150 and 1152 would mappixels in data shift register cells to a corresponding list of pixelsthat define the members of the mapped composite pixel/superpixel).However, it should also be understood that this one-to-many mapping maybe performed by control logic upstream from the data shift register1170, in which case the data shift register may comprise multiple cellsin parallel that are populated with the members of the compositepixel/superpixel for each targeted pixel from the shot list 800.

FIG. 12 depicts an example process flow for implementing adaptivecontrol techniques for controlling how the receiver adapts the activeregion of the photodetector array 600. At step 1200, a list of pixelseligible for inclusion in subset 1130 is defined. This list can be anydata structure 1202 that includes data indicative of which pixels 602are eligible to be selected for inclusion in the subset 1130. Such adata structure may be maintained in memory that is accessible to aprocessor that implements the FIG. 12 process flow. While the example ofFIG. 12 shows a list 1202 that identifies eligible pixels 602, it shouldbe understood that data structure 1202 could also serve as an effectiveblacklist that identifies pixels that are ineligible for inclusion insubset 1130.

At step 1204, a circuit (e.g., signal processing circuit 606 and/orcontrol circuit 608), which may include a processing logic (e.g., anFPGA) and/or other processor, operates to derive information from thelight sensed by the array 600 (which may be sensed by a subset of pixels602 that are active in the array) or from the environmental scene (e.g.,by processing camera/video images). This derived information may includeinformation such as whether any saturation conditions exist, whether anypixels are malfunctioning, whether there are any areas of high noise inthe field of view, etc. Examples of derived information that can beuseful for adaptive control are discussed below. Furthermore, it shouldbe understood that the oversaturation conditions can be attributed tospecific pixels (e.g., pixels that are blinded by intense incidentlight) and/or can be attributed to the aggregated signal resulting fromthe combination of pixel readings by the pixels included in subset 1130(where the aggregation of pixel outputs oversaturates the linearoperating range of the processing circuitry).

At step 1206, the list of eligible pixels 1202 is adjusted based on theinformation derived at step 1204. For example, if a given pixel is foundto be malfunctioning as a result of step 1204, this pixel can be removedfrom list 1202 at step 1206. Similarly, any oversaturated pixels can beremoved from the list 1202 and/or any pixels corresponding to overlynoisy areas of the field of view (e.g., regions where the noise exceedsa threshold) can be removed from list 1202 at step 1206.

Next, at step 1208, the system selects pixels from the list 1202 ofeligible pixels based on the targeted range point. This can be performedas described in connection with step 804 of FIG. 8, but where list 1202defines the pool of pixels eligible to be selected as a function of thelocation of the targeted range point in the scan area/field of view. Itshould be noted that this function can take into consideration a numberof different parameters and variables. For example, off-axis the beamaberration will stretch the size of this set. As another example, atnear range, parallax may encourage the inclusion of additional pixels.Thus, if the targeted range point is mapped to pixel 1140 in the arrayand the subset 1130 would have ordinarily included all of the pixelsthat neighbor pixel 1140, the adaptive control technique of FIG. 12 mayoperate to define subset 1130 such that the upper left neighboring pixelof pixel 1140 is not included in subset 1130 if the upper leftneighboring pixel was removed from list 1202 at step 1206 (e.g., due toa detected malfunction or the like). Furthermore, it should beunderstood that step 1208 may also operate to use the informationderived at step 1204 to affect which eligible pixels are included in thesubset. For example, additional pixels might be added to the subset 1130to increase the size of the active sensor region based on the derivedinformation. Similarly, the size of the active sensor region might beshrunk by using fewer pixels in the subset 1130 based on the derivedinformation. Thus, it should also be understood that the size of theactive region defined by the selected subset 1130 may fluctuate fromshot to shot based on information derived at step 1204.

FIG. 17 shows examples of different shapes of pixel subsets that may bechosen for the active sensor region. A pixel selected for inclusion inthe pixel subset 1130 is shown in heavy shading while pixels that arenot included in the pixel subset 1130 are shown with no fill. Subset1702 shows an example where a single pixel forms the subset 1130. Thissingle pixel can be the specific pixel that was mapped to as a result ofthe targeted range point. Pixel mask 1702 corresponds to the case whereone seeks to reduce the amount of light received by the signalprocessing circuit—which can be a typical instantiation at short range.Subset 1704 shows an example where the subset 1130 includes the mappedpixel as a center pixel plus the pixels surrounding the center pixel.Pixel mask 1704 can correspond to a standard selection for the system.Subsets 1706, 1708, 1710, and 1712 show different examples of shapes foractive sensor regions that might be used. Pixel mask 1706 can be used incases of beam aberration and/or parallax for near range. Pixel masks1708 and 1712 can be used as halo shape, which can be useful for variousexample embodiments discussed below. Pixel mask 1710 can useful in caseswhere a specific pixel is found to be malfunctioning (also discussedbelow). Moreover, it should be understood that each pixel shown by FIG.17 could itself be a superpixel of individual pixels if desired by apractitioner.

At step 1210, the pixels selected at step 1208 are included in subset1130, and the MUX is then controlled to read/combine the outputs fromthe pixels that are included in the selected subset 1130 (step 1212).Thereafter, the process flow returns to step 1204 for the next ladarpulse shot. Accordingly, it can be seen that the process flow of FIG. 12defines a technique for intelligently and adaptively controlling whichpixels in array 600 are used for sensing ladar pulse returns.

Furthermore, it should be understood that the FIG. 12 process flow canalso be used to impact transmitter operation. For example, the list ofeligible pixels (or a list of ineligible pixels) can be provided to theladar transmitter for use by the ladar transmitter to adjust thetiming/order of shots on its shot lists (e.g., avoiding shots that willlikely be corrupted by noise on receive). Further still, as an example,if the information derived at step 1204 indicates that the aggregatedsignal produced by MUX 604 is oversaturated, the ladar transmitter canreduce the power used by the ladar pulses 108 to reduce the likelihoodof oversaturation on the receive side. Thus, when such oversaturationcorrupts the receiver, the ladar transmitter can repeat the corruptedshot by reducing the power for ladar pulse 108 and re-transmitting thereduced power pulse.

Also disclosed herein specific examples of control techniques that canbe employed by the ladar system. While each control technique will bediscussed individually and should be understood as being capable ofimplementation on its own, it should also be understood that multiplesof these control techniques can be aggregated together to furtherimprove performance for the adaptive receiver. As such, it should beunderstood that in many instances aggregated combinations of thesecontrol techniques will be synergistic and reinforcing. In other cases,tradeoffs may exist that are to be resolved by a practitioner based ondesired operating characteristics for the receiver. As a representativeexample, dual channels can be fed into a back-end ADC with differentgain settings, to allows weak signals to be observed while avoidingsaturation on strong signals. Likewise, we can select or de-selectpixels based on considerations of the pixel-dependent expected powerlevels from parallax.

Adaptive Fault Tolerance Mask:

With a conventional imaging array, a dead pixel typically leads toirrecoverable loss. However, with the adaptive control featuresdescribed herein, a malfunctioning pixel 602 has minimal effect. Supposefor example that we have an array 600 of 500 pixels 602. Then suppose wehave a lens that maps the far field scene to a 7-pixel super/compositepixel 1130 (a specified pixel 1140 and its neighbors). Losing one pixelleads to a loss of 1/7 of the net photon energy. If the detector arrayis shot noise-limited, then we have only a 7% loss in energy, versus100% loss for a full imaging array. An example control flow for a faulttolerant adaptive mask is shown below as applied to an embodiment wherethe ladar transmitter employs compressive sensing. It should beunderstood that a mask can be used by the control circuit 608 to definewhich pixels 602 are included in the selected subset of active sensorsand which are not so included. For example, the mask can be a datasignal where each bit position corresponds to a different pixel in thearray 600. For bit positions having a value of “1”, the correspondingpixel 602 will be included in the selected subset, while for bitpositions having a value of “0”, the corresponding; pixel 602 would notbe included in the selected subset.

A pixel 602 that is unable to detect light (i.e., a “dead” pixel or a“dark” pixel) should not be included in the selected subset because sucha dead pixel would add noise but no signal to the aggregated sensedsignal corresponding to the composite pixel defined by the selectedsubset. Furthermore, it should be understood that malfunctioning pixelsare not limited to only dead pixels. A pixel 602 that produces an outputsignal regardless of whether incident light is received (e.g., a “stuck”pixel or a “white” pixel) should also be omitted from the selectedsubset. In fact, a white pixel may be even worse than a dark pixelbecause the stuck charge produced by the white pixel can lead to aconstant bright reading which adds glare to all returns in the compositepixel. An example control process flow is described below for generatingan adaptive fault tolerant mask that can adjust which pixels 602 areincluded in the selected subset based on which pixels 602 are detectedas malfunctioning:

-   -   1: select a background pixel status probe shot schedule        repetition rate T (e.g., nominally one per hour).    -   2: Decompose: In the past previous time block T identify S, the        set of pixels not yet selected for illumination. Decompose into        S1, S2, the former being addressable (strong return in scene)        while the latter is defined to be non-addressable (ex: above        horizon). Note that S1, S2 are time varying.    -   3: Shot list: Enter S1, S2 into the shot list.    -   4: construct a mask to deselect faulty tiles identified from        analysis of returns from 1-3 (either no return or anomalous        gain). The super-pixel size can be set based on the lens and        tile pitch but can nominally be 7.    -   5: recurse 1-4.    -   6: average: In the above, as necessary, apply running averages        on pixel probe, and include adaptive metrology.

Fault tolerance in this fashion can be a useful step in improvingsafety, since without mitigation single defects can render an entire FOVinoperative.

Adaptive Mask to Control Dynamic Range:

The adaptive control over which subsets of pixels are activated at agiven time can also be used to adjust the dynamic range of the system.Based on range knowledge, the signal produced by a composite pixel willhave predictable intensity. A mask can be constructed that reduces (orincreases) the dynamic range of the return at the ADC pre-filter and/orthe ADC itself by adjusting the size of the composite pixel defined bythe pixels 602 included in the selected subset. For example, if thetypical composite pixel is 7 pixels (see 1130 in FIG. 11B), adjustingthe subset such that it drops from 7 pixels to a single pixel reducesthe energy by 7 times (or roughly three bits). Photodetectors measureenergy of light, not amplitude of light. As a result, the ADC's dynamicrange is the square of that for conventional communications and radarcircuits which measure amplitude. As a result, properly controllingdynamic range is a technical challenge for laser systems. For example, aladar system tuned to operate over 10-500 m will, for a fixedreflectivity and laser power, undergo a signal return dynamic rangechange by 2500. If a nearby object saturates the receiver, a farther outtarget will be lost. Therefore, an example embodiment can include ananalysis of prior shot range returns in the instantaneous field of viewto assess the need to excise any pixels from the selected subset in themux circuit. As a result, there may be a desire for having the MUX dropthe sensor signal(s) from one or more pixels of the region 1130, asoutlined below. An example control process flow is described below forgenerating an adaptive mask for controlling the dynamic range of thereturn signal:

-   -   1. Inspect range return from a pulse return of interest,        obtained from either selective or compressive sensing.    -   2. Identify any saturation artifacts, as evidenced by ADC        reports at the MSB (most significant bit) for several range        samples.    -   3. Map the saturated range sample to a precise azimuth and        elevation of origin. This may involve exploring adjacent cells        to determine origin from context, particularly at longer range        as beam divergence is more pronounced.    -   4. Modify the mask to reduce saturation by blocking the pixels        that present a larger gain the origin identified in 3.    -   5. Modify the mask further by selecting only smaller area pixels        as required.

Adaptive Mask to Remove Interfering Ladar Pulse Collisions:

Another potential source of noise in the light sensed by the receiver isa collision from an interfering ladar pulse. For example, in anapplication where the ladar system is employed on moving automobiles,the incoming light that is incident on the photodetector array 600 mightinclude not only a ladar pulse return 112 from the vehicle that carriesthe subject ladar system but also a ladar pulse or ladar pulse returnfrom a different ladar system carried by a different vehicle (aninterfering “off-car” pulse). Adaptive isolation of such interferingpulses can be achieved by creating a sub-mask of selected pixels 602 byexcising pixels associated with strong interfering pulses from otherladar systems. Alternately, one can create a “halo” pixel mask, (see,for example 1708 and 1712 in FIG. 17), centered around the range point.Without the presence of interference, this halo can be expected tocontain no laser energy, but in the presence of an interfering laser, itcan be expected to possess energy.

The above-referenced and incorporate patent applications describe howpulse encoding can be employed to facilitate the resolution as to whichladar pulses are “own” pulses and which are “off” pulses (e.g.,“off-car” pulses). For example, consider that such encoding is used todetect that pixel 1134 contains energy from an interfering ladar pulse.We would then scan through the pixels of the array (with the cluster1130 for example) to see which are receiving interference. In oneembodiment, this would involve removing the “own” lidar pulse usingencoding, measuring the resulting signal after subtraction, andcomparing to a predetermined threshold. In another embodiment, thesystem would simply analyze the MUX output, subtract off the “own” pulseencoding signal and compare the remainder to a threshold. The embodimentwill depend on the severity of interference encountered, and processorresources that are available. Upon such detection, the control circuit608 can remove this pixel 1134 from a list of eligible pixels forinclusion in a selected subset while the interfering pulse is registeredby that pixel 1132.

The system might also remove pixels based on headlight sourcelocalization from passive video during night time operations (theoperational conservative assumption here being that every vehicle with aheadlight has a ladar transmitter). Furthermore, since pulse collisiondetection can be used to reveal off-car pulses, this information can beused to treat any selected off car laser source as a desired signal,subtract off the rest (including own-car ladar pulses) and scan throughpixels of the array to find where this interference is largest. In doingso we will have identified the source of each interfering ladar source,which can then be subsequently removed.

Adaptive Mask for Strong Scatterer Removal:

Another potential source of noise in the light sensed by the receiver iswhen a ladar pulse strikes an object that exhibits a strong scatteringeffect (e.g., a strongly slanted and reflective object as opposed to amore ideally-oriented object that is perpendicular to the angle ofimpact by the ladar pulse 108). Targets exhibiting multiple returns haveinformation bearing content. However, this content can be lost due toexcessive dynamic range, because the largest return saturates drivingthe receiver into nonlinear modes, and/or driving the weaker returnsbelow the sensor detection floor. Typically, the direct return is thelargest, while successive returns are weakened by the ground bouncedispersion, but this is not the case when reflectivity is higher inbounce returns. in either case, it is desirable to adjust the mask sothat the near-in range samples receive a. higher pupil (dilation) (e.g.,where the selected subset defines a. larger area. of the array 600),while the farther out range samples undergo pupil contraction (e.g.,where the selected subset defines a smaller area of the array 600). Atfar range there will be large angular extent for the laser spot. Itispossible for strong near-range scatterer pulse returns to arrive withinthe data acquisition window for the transmitted pulse. The use of anadaptive mask will allow for the removal of this scatterer byover-resolving the spot beam (e.g., more than one pixel covered by theshot return beam) on receive, thereby reducing saturation or scattererleakage into the target cell. For example suppose, notionally we observethat the range returns begin at 1134, migrate to the doublet at 1132 andat closest range appear at 1130. We can then instruct the controlcircuit to modify the mask by choosing different mux lines as the laserpulse sweeps across the sensor array.

Adaptive Shot Timing Linked to Mask Feedback Control:

In compressive sensing, the dynamic range can be further reduced bydeliberately timing the laser pulse by the transmitter so that the laserpeak intensity does not fall on the target but instead falls away fromnear-to-the-target interference, thereby increasing the signal toclutter ratio. This allows for near-in interference suppression aboveand beyond that obtained by other means. For example, suppose,notionally, that the upper sensor cell 1132 contains a very strongtarget and the lower nearby sensor cell also labeled 1132 contains atarget. Then we can set the shot timing to move the received pulse shotillumination away from the 1132 doublet and center it more towards 1130.We are using here the flexibility in shot timing (provided viacompressive sensing), knowledge of beam pointing on transmit (see FIG.9), and selectivity in sensor elements (see FIG. 11B, for example) tooptimally tune the receiver and transmitter to obtain the best possiblesignal quality. By ensuring the mask is tuned so that the beam peak ofthe receive beam is away from a noise source (e.g., incoming traffic) wecan reduce strong returns from nearby vehicles while imaging atdistance, a milliradian in some cases suffices to reduce strongscatterers by 95% while attenuating the target object by only a fewpercent. In an example embodiment, selective sensing can be used todetermine the mask parameters, although compressive sensing, or fixedroadmap-based solutions may also be chosen. An example here is lanestructure, since opposing lane traffic yields the largest interferencevolume. The system could thereby adjust the shots, or the ordering ofshots to avoid noisy areas while retaining the desired objectinformation.

Adaptive Mask for Dynamic Range Mitigation by Mask Mismatch:

If the mask in 1130 is chosen to provide the largest ladar reflectionmeasurement, the center pixel will have the most energy. Therefore itwill saturate before any of the others. Therefore one approach forreducing saturation risk is to simply remove the center pixel from themask 1130 if evidence of, or concern regarding, saturation is present(see, for example, 1712 in FIG. 17).

Adaptive Mask for Power-Coherent Interference Rejection:

One benefit of the advanced receiver disclosed herein is that only asingle data channel is needed, as opposed to M where M is the pixelcount. However, one can still retain a low cost and swap system byadding a second channel. This second channel, like the first channel,can either be a full up analog to digital converter (see FIG. 7A) or atime of flight digitizer (see FIG. 7B). Either embodiment allows forcoherent combining (in intensity) to optimally suppress the interferenceusing filtering (such as Weiner Filtering or Least Means Squared (LMS)Filtering). With two channels c1,c2 and with the target return weightingbeing w_(c1), w_(c2), this is equivalent to solving for the weights andapplying the weights to the data so that the SNR of w_(c1)c1+w_(c2)c2 ismaximized. Through such an adaptive mask, the spatially directionalnoise component in the sensed light signal can be reduced.

FIG. 18 depicts an example of a multi-channel receiver where tworeception channels are employed. In this example, each channel includesits own multiplexer 604 to control which subsets of pixels in thedetector array 600 will be read out. Multiplexer 604 ₁ is for Channel 1,and multiplexer 604 ₂ is for Channel 2. Each channel's multiplexerreceives the same input signals 610 from the photodetector pixels.Control circuit 608 can generate separate control signals 612 for eachchannel multiplexer (see control signal 612 ₁ for Channel 1 and controlsignal 612 ₂ for Channel 2). With reference to FIGS. 12 and 17, thecontrol circuit 608 can process the shot list 800 to identify thephotodetector pixel that maps to the targeted range point and thenselect different pixel mask shapes with reference to that mapped pixelfor the two channels. For example, the control circuit 608 can providechannel-specific control signals 612 to the multiplexers that areeffective to select pixel mask shape 1704 for the Channel 1 multiplexer604 ₁ and pixel mask shape 1708 for the Channel 2 multiplexer 604 ₂.With such an approach, the Channel 1 MUX output signal 1802 would bedifferent than the Channel 2 MUX output signal 1804.

The signal processing circuit 606 could then process the two differentchannel output signals 1802 and 1804 to support assessments as towhether the incident light on the photodetector 600 corresponds tointerference or a ladar pulse return. As discussed above, it is to beexpected that a ladar pulse return will have its energy overwhelminglyconcentrated in the region of pixel mask 1704 but not in the region ofpixel mask 1708. Thus, signal processing circuit 606 can assess whetherthe incident light on the region of interest for the array 600corresponds to a ladar return or interference by computing and comparingmeasures that are indicative of the strength of signal 1802 versussignal 1804. The signal processing circuit 606 can accomplish this usingestablished methods from hypothesis testing. For example, under thehypothesis of own-laser only, the first channel signal 1802 will have anenergy level dictated by range, transmit laser energy, and targetreflectivity. Likewise, under this hypothesis, the second channel signal1804 will have an energy level dictated by noise sources includingbackground shot noise. By comparing data on intensity from 1802 and 1804to those predicted by the no-laser-interference hypothesis, and flaggingoutliers, one can perform the above comparison. The above exampleinvolves the use of dual output channels for detecting laserinterference, but it can also be used to obtain wider dynamic range byusing both channels with the same pixel set and applying both a high andlong gain to each channel respectively.

While FIG. 18 shows an example where the receiver includes twoseparately-controllable readout channels, it should be understood thatadditional separately-controllable readout channels could be employed aswell if desired. For example, different channels can be controlled toinclude a number of different pixel mask shapes, and post-processing bythe signal processing circuit 606 can be used to decide which channeloutput signal (or combination of channel output signals) can best beused to support range measurements for ladar pulse returns. In stillanother example, different receiver channels can be used to read out oneor more regions of the array 600 outside the region of interest for thetargeted range point so that the signal processing circuit can assessnoise and interference conditions that may be present elsewhere in theenvironmental scene. With a design such as this, the signal processingcircuit 606 can be used to detect and identify other ladar transmittersthat are transmitting ladar pulses in the vicinity of the subjectsystem. One can also use this approach to inspect the pixel(s) staringat the sun to determine how much solar-included noise we obtain, whichcan be useful information in performance prediction such as determiningmaximum range.

The embodiments of FIGS. 6A-12 and 18 can be particularly useful whenpaired with detection optics such as those shown by FIGS. 4 and 5A,where the sensed light is imaged onto the detector array 600. Inembodiments where the image pulse is not imaged onto the detector array600 (e.g., the embodiments of FIGS. 3A, 3B, and 5B (or embodiments wherethe image is “blurry” due to partial imaging), then a practitioner maychoose to omit the multiplexer 604 as there is less of a need to isolatethe detected signal to specific pixels.

FIG. 13A depicts an example ladar receiver embodiment where “direct todetector” detection optics such as that shown by FIG. 5A are employedand where the readout circuitry of FIG. 7A is employed. In this example,the ladar receiver is designed with an approximately 60×60 degree FOV,and an approximate 150 m range (@SNR=8, 10% reflectivity). The receiveremploys a low number N-element detector array such as an InGaAs PIN/APDarray. When using an InGaAs PIN array, the receiver may exhibit a 2 cminput aperture, a 14 mm focal length, and it may work in conjunctionwith an approximately 0.2-5.0 nanosecond laser pulse of around 4microJoules per pulse. Spatial/angular isolation may be used to suppressinterference, and a field lens may be used to ensure that there are no“dead spots” in the detector plane in case the detectors do not have asufficiently high fill factor. FIG. 13B depicts a plot of SNR versusrange for daytime use of the FIG. 13A ladar receiver embodiment. FIG.13B also shows additional receiver characteristics for this embodiment.Of note, the range at reflectivity of 80% (metal) is over 600 m.Furthermore, the max range envelope is between around 150 m and around600 m depending on real life target reflectivities and topography/shape.

FIG. 14A depicts an example ladar receiver embodiment where detectionoptics such as that shown by FIG. 3B are employed and where the readoutcircuitry of FIG. 7A is employed. In this example, the ladar receiver isdesigned with an approximately 50×50 degree FOV, and an approximate 40 mrange (@SNR=8, 10% reflectivity). As with the embodiment of FIG. 13A,the receiver employs a low number N-element detector array such as a PINphotodiode array, e.g. an InGaAs PIN/APD array. When using an InGaAs PINarray, the receiver of FIG. 14A may exhibit a 2 cm input aperture,employ an afocal non-imaging lens, and it may work in conjunction withan approximately 0.2-5.0 nanosecond laser pulse of around 4 microJoulesper pulse. FIG. 14B depicts a plot of SNR versus range for daytime useof the FIG. 14A ladar receiver embodiment. FIG. 14B also showsadditional receiver characteristics for this embodiment. Of note, therange at reflectivity of 80% (metal) is around 180 m. Furthermore, themax range envelope is between around 40 m and around 180 m depending onreal life target reflectivities and topography/shape.

It is also possible to dramatically improve the detection range, the SNRand therefore detection probability, or both, by exploiting motion ofeither a ladar system-equipped vehicle or the motion of the objects itis tracking, or both. This can be especially useful for mapping a roadsurface due to a road surface's low reflectivity (˜20%) and the pulsespreading and associated SNR loss.

The stochastic modulation of the two way (known) beam pattern embedsposition information on the point cloud(s) obtained. We can extract fromthis embedding improved parameter estimates. This is essentially thedual of ISAR (inverse synthetic aperture radar) in radar remote sensing.This is shown in FIG. 15, where we show the detector output for a givenazimuth and elevation pixel, with each row being the range returns froma single shot. As we aggregate shots we obtain integration gain. In FIG.15 the solid white curve 1502 shows how a specified, fixed, groundreference point varies vertically due to vehicle motion. Note that themotion can lead to a non-linear contour. This is due to the fact that,even for fixed velocity, the ground plane projection does not, at nearrange, present a planar projection. In other words, the Jacobian of theground plane projection is parametrically variant. The relative motionexploitation that we propose is to integrate the detector array outputs,either binary or intensity, along these contours to recreate the groundplane map. Such integration is necessitated in practice by the fact thatthe pulse spreads and thus each shot will present weak returns. Further:the asphalt tends to have rather low reflectivity, on the order of 20%,further complicating range information extraction. The white rectangularregion 1502 show the migration in shots for a vehicle presentingrelative motion with respect to the laser source vehicle. To simplifythe plot, we show the case where differential inter-velocity [closingspeed] is constant. The width of the rectangle 1502 presents theuncertainty in this differential. The scale shows this width is muchlarger than the width for ground mapping described above. This isbecause we must estimate the differential speed using ladar, while theown-car has GPS, accelerometers, and other instrumentation to enhancemetrology. Close inspection will show that inside the white tiltedrectangle 1502 there are more detections. This example is for an SNR of2, showing that, even at low SNR, an integration along track [binary]can provide adequate performance. The receiver operating curve can bereadily computed and is shown in FIG. 16. Shown is the detectionprobability, 1600 (thin lines upper right) as well as the false alarmcurve, bottom left, 1602. We move for thin lines from one shot to 30shots. The horizontal axis is the threshold at the post integrationlevel, forming lines in the kinematic space as per FIG. 15. At athreshold of 1.5 observe that we get 95% 6% Pd Pfa at 15 shots, whichfor a closing speed of 50 m/s is 25 m target vehicle ingress, or ½second.

While the invention has been described above in relation to its exampleembodiments, various modifications may be made thereto that still fallwithin the invention's scope. Such modifications to the invention willbe recognizable upon review of the teachings herein.

What is claimed is:
 1. A ladar receiver apparatus comprising: an arraycomprising a plurality of photodetectors; a plurality of amplifiers,each of a plurality of the amplifiers configured to amplify a signalproduced by a corresponding photodetector such that different amplifiersamplify signals produced by different corresponding photodetectors; anda circuit configured to (1) process a shot list that identifies aplurality of range points for targeting by ladar pulse shots over time,(2) select a plurality of different subsets of the photodetectors forread out over time based on the range points of the processed shot list,(3) selectively control how power is applied to the amplifiers over timebased on the range points of the processed shot list, (4) process theamplified signals from the amplifiers corresponding to the selectedsubsets of photodetectors, and (5) compute range information for therange points targeted by the ladar pulse shots based on the processedamplified signals.
 2. The apparatus of claim 1 wherein the circuit isfurther configured to (1) power down the amplifiers for a first timeperiod where their corresponding photodetectors are not in the selectedsubset for signal readout, and (2) power up the amplifiers for a secondtime period where their corresponding photodetectors are in the selectedsubset for signal readout.
 3. The apparatus of claim 2 wherein thepowered down amplifiers are in a quiescent state.
 4. The apparatus ofclaim 2 wherein the circuit is further configured to control the secondtime period such that the second time period begins before the signalreadout from the selected subset of the corresponding photodetectors. 5.The apparatus of claim 2 wherein the circuit is further configured toapply shot pipelining to the shot list to control the subset selectionsand how the amplifiers are powered.
 6. The apparatus of claim 5 whereinthe circuit comprises: a multiplexer configured to select thephotodetector subsets; a register comprising a plurality of cellsthrough which data about the ladar pulse shots of the shot list arestreamed; amplifier control logic configured to (1) read data from afirst cell of the register, and (2) select the amplifiers for poweringup based on the read first cell data; and multiplexer selection controllogic configured to (1) read data from a second cell of the register,wherein the second cell is downstream from the first cell with referenceto a stream direction for the streaming ladar pulse shots data, and (2)control the multiplexer to select the photodetector subsets based on theread second cell data.
 7. The apparatus of claim 6 wherein amplifiercontrol logic is further configured to power down a powered up amplifierafter the amplified signal from the powered up amplifier has been readout and processed.
 8. The apparatus of claim 1 wherein the amplifierscomprise transimpedance amplifiers.
 9. The apparatus of claim 1 whereinthe circuit provides a controlled feedback loop for adjusting gains ofthe amplifiers.
 10. The apparatus of claim 1 wherein each photodetectorhas a different corresponding amplifier.
 11. A method comprising:processing a shot list, wherein the shot list comprises data about aplurality of ladar pulse shots for targeting a plurality of rangepoints; identifying a ladar pulse shot from the shot list; transmittinga ladar pulse in accordance with the identified ladar pulse shot;selecting a subset of a plurality of photodetectors based on theidentified ladar pulse shot; selecting a subset of a plurality ofamplifiers based on the identified ladar pulse shot; powering up theselected subset of amplifiers; the selected subset of photodetectorsgenerating a signal that is representative of a reflection of thetransmitted ladar pulse; the selected and powered up subset of theamplifiers amplifying the generated signal; processing the amplifiedsignal to determine range information for the range point targeted bythe identified ladar pulse shot; and repeating the method steps for aplurality of different ladar pulse shots on the shot list.
 12. Themethod of claim 11 further comprising: for each identified ladar pulseshot, powering down a plurality of the amplifiers that are not includedin the selected subset of amplifiers for that identified ladar pulseshot.
 13. The method of claim 12 wherein the powering down stepcomprises, for each identified ladar pulse shot, controlling a pluralityof the amplifiers that are not included in the selected subset ofamplifiers for that identified ladar pulse shot to be in a quiescentstate.
 14. The method of claim 11 wherein the shot list processing stepcomprises streaming the ladar pulse shots data through a pipeline, themethod further comprising: performing the method steps based on aplurality of reads of the streaming ladar pulse shots data from thepipeline.
 15. The method of claim 14 wherein the pipeline comprises aregister, the register comprising a plurality of cells, and wherein thestreaming step comprises streaming the ladar pulse shots data throughthe cells over time such that each of a plurality of the cells comprisesdata for a different ladar pulse shot from the shot list; wherein thestep of selecting amplifier subsets comprises (1) reading ladar pulseshots data from a first cell of the register as the ladar pulse shotsdata streams through the cells, and (2) selecting the amplifier subsetsbased on the read ladar pulse shot data from the first cell as the ladarpulse shots data streams through the cells; and wherein the step ofselecting photodetector subsets comprises (1) reading ladar pulse shotsdata from a second cell of the register as the ladar pulse shots datastreams through the cells, and (2) selecting the photodetector subsetsbased on the read ladar pulse shots data from the second cell as theladar pulse shots data streams through the cells, wherein the secondcell is downstream from the first cell with reference to a streamdirection for the streaming ladar pulse shots data.
 16. The method ofclaim 11 wherein the amplifiers comprise transimpedance amplifiers. 17.The method of claim 11 further comprising adjusting gains for theamplifiers using a controlled feedback loop.
 18. The method of claim 11wherein each photodetector has a different corresponding amplifier foramplifying the signals generated by that photodetector.
 19. The methodof claim 11 wherein the step of selecting photodetector subsetscomprises controlling a multiplexer to pass the amplified signal derivedfrom each photodetector in the selected photodetector subset.
 20. Themethod of claim 11 wherein the transmitting step comprises transmittingthe ladar pulses in accordance with the identified ladar pulse shots viaa plurality of scanning mirrors that are controlled to providecompressive sensing.